Ion sources have proven useful for the modification of substrates and for the deposition of thin films. The energetic ions serving to etch surfaces and provide energy to a growing film. A challenge confronting the industrial use of ion sources is the tendency for the anode to become contaminated by the process byproducts, particularly when those byproducts result in an insulating film forming on the anode. When the anode is coated by an insulating film, electrons can no longer effectively reach the anode and ion source operation is disrupted. The present invention relates to improvements in closed drift ion sources. Closed drift ion sources should not be confused with other types of ion sources that include gridded ion sources and ion sources with axial or mirror magnet field configurations. Gridded ion sources use a series of electrified grids to accelerate ions from a plasma discharge cavity. Multiple power supplies are required to operate a gridded ion source and ion source construction, with precision grids, makes these ion sources complex and expensive. Additionally, thermal expansion considerations of the grids makes extending these sources for use with large substrates impractical. Closed drift ion sources expose the ion accelerating anode to the process chamber and intervene an orthogonal magnetic field between the anode and the process chamber. Ion creating electrons are injected into the chamber and must cross the magnetic field lines to reach the anode. In encountering the magnetic field, the electrons drift in the Hall direction and by configuring the source to provide a closed racetrack, the electrons are effectively trapped in an endless loop. Side walls, floating or grounded and forming a channel between the process chamber and anode, block electron flow along magnetic field lines.
Closed drift ion sources have specific advantages that make them commercially successful: 1) By forcing electrons to cross magnetic field lines to reach the anode, a strong impedance to electron current is created that sets up an ion accelerating electric field of hundreds of volts. The resulting ion beam emanating from a closed drift ion source is energetically useful for a number of processes. This crossed magnetic field impedance is greater than the impedance across an axial or mirror magnetic field. 2) the Hall drift caused by orthogonal electric and magnetic fields tends to create a uniform electron current around the closed loop racetrack and a uniform ion beam emanates from the racetrack. This effect is very useful for treating large substrates as the racetrack can be extended over several meters.
The prior art has divided closed drift ion sources into three classifications: extended acceleration channel, anode layer types, and end Hall. Though not completely consistent, the general distinguishing factor between extended acceleration channel and anode layer types is the ratio of the electron confining channel width to the channel depth. If the channel depth exceeds the channel width dimension, the ion source is termed an extended acceleration channel type. Representative of an extended acceleration channel type of closed drift ion sources is U.S. Pat. No. 5,646,476. Prior art extended acceleration channel type sources have an anode that is positively charge biased.
Anode layer type ion sources are the second type of closed drift source. In an anode layer type source, the closed channel depth is typically shorter or equal to the width. Representative configurations of the anode layer type of closed drift ion sources are U.S. Pat. Nos. 5,763,989; 5,838,120 and 7,241,360. U.S. Pat. No. 7,241,360 is characterized by having the entire ion source biased charge negative on the negative AC cycle to sputter clean the ion source.
End Hall ion sources represent a variation of a closed drift ion source. In the end Hall source, the inner magnet pole is lower with respect to the outer pole to expose the sides of the annular anode. With this geometry, a second electron confinement regime combines with the Penning style confinement of closed drift ion sources. The second confinement regime is mirror electron confinement in which electrons are partially confined along magnetic field lines by a gradient magnetic field. Representative of such sources are U.S. Pat. Nos. 6,750,600 and 6,870,164. In particular, U.S. Pat. No. 6,750,600 attempts to reduce the loss of anode conductivity associated with contaminant deposition on the anode. U.S. Pat. No. 6,870,164 addresses anode degradation through contaminant deposition thereon by applying a positive charge bias in pulses to the anode to avoid operational instabilities, yet does not bias the anode negative such that a plasma is generated on the anode.
Numerous approaches have been taken to address the problem of anode contamination for all types of closed drift ion sources. In spite of this, there remains a need for a process and apparatus for closed drift ion source that can consistently operate without losing anode conductivity over time. Ideally, a solution would include a self-cleaning anode that remained free from insulating buildups even over extended process times of many 10's of hours.